Acoustic delay line using solid rods



.am f f I //'!l' ",;fl4"wk, ma 2,727,211, ff/MQ f "Nv'l RLLRL CROSS Dec.13, 1955 H. J. MCSKIMIN ACOUSTIC DELAY LINE USING SOLID RODS Filed Nov.2, 1949 .I q l on 1w 3 35,/ 3 me 3 Sheets-Sheet 1 /NVENTOR H. J.MCSK/M/N ATTORNEY Dec. 13, 1955 H. .1.A MosKlMlN ACOUSTIC DELAY LINEUSING soun Hons 5 Sheets-Sheet 2 Filed NOV. 2, 1949 3RD HARHON/L' OPERAT/ON 60 HEGACYCLES ATTORNEY Dec. i3, 1955 H. J. MCSKIMIN ACOUSTIC DELAYLINE USING SOLID RODS 3 Sheets-Sheet 3 Filed Nov. 2, 1949 wtx auf A L Ewm .N CDE M my .P Umm mw. c Ro rm, G .m L m C LR F `s mm. n 6M u NII l lI l i I l I l I Il M TW N0 C www 9 V. w 2 y 7 l? 2 QVY.. ,w m 0. F l'\||4\ E@ values for other liquids.

delay line, however, have beenY unsuccessful, for reasons nite aresarent ACOUSTIC DELAY LlNE USING SOLlD RODS Herbert J. McSirimin, BaskingRidge, N. J., assigner to Bell Telephone Laboratories, incorporated, iTcw York, N. Y., a corporation of New York Application November 2, 1949,Serial No. 125,049

8 Claims. (Cl. S33- 30) This invention relates to delay lines, andparticularly to a line utilizing a solid rod as the medium for thetransmission of ultrasonic vibrations.

It is an object of the invention to provide an improved form of delayline in which the means for effecting retardation include a solid wavetransmission member.

The delay line is an instrument which retards an electrical signal for aknown period, and then delivers it for subsequent use in a circuit. Inmost cases it is desirable that the delayed pulse be delivered withoutsubstantial distortion of the wave form. Delay lines nd such diverseapplications as in moving target indicators for radar use, in theproduction of more natural tones in broadcasting from sound proofedstudios by simulating the echoes normally produced in an auditorium, andin providing the memory in electrical devices for solving complexproblems.

In the delay line disclosed in this application, the electrical signalto be delayed is impressed on a piezoelectric crystal transducer,frequently operating in the megacycle range because of the band-widthrequirements. This sets up an ultrasonic mechanical vibration in thecrystal, which is communicated to a transmitting medium of known length.After passage through the transmitting medium, theV ultrasonicvibrations are impressed on a receiving piezoelectric crystal, whichconverts them back into electrical impulses corresponding to theoriginal signal. The velocity of propagation of the ultrasonicvibrations is so small, compared with that of electrical impulses, thata signal reaching a certain point in the circuit through whollyelectrical paths will arrive a substantial length of time before onereaching that point through a path including a mechanically conductingelement.

Liquids have been used successfully as the medium for transmittingultrasonic vibrations, and particularly mercury, which has a very lowloss factor. For certain applications, however, such as portable uses, asolid delay line wouldV be superior. to rough held use because of theproblems presented by its weight and the fragility of the crystals, andbecause of the ditculty of preventing contamination of the mercury andother components. Even minute amounts of dissolved or trapped air,moisture, and dirt, may cause serious intreference with the properfunctioning of such a line. A solid line is superior in ruggedness andease of maintenance to a liquid line. The solid line is also superior inregard to its acoustic attenuation characteristics, the loss varyingwith the first power, rather than the square` of the frequency-forfrequencies at which scattering is negligible-so that operation may beat higher frequencies. and compensating networks, if required at all,will be less complicated. Temperature stability is also superior in thesolid line, fused silica having a temperature coefficient of delay ofabout- 70 parts per million per degree centigrade, as compared with 340for mercury, and higher Prior attempts to make a solid whichV willappear more fully in the discussion of the various features of thepresent invention.

The present invention lies in the discovery of means for making and.using a solid line successfully: it includes among its features foldingthe line without destroying The mercury line is not well suitedice theshape otthe transmitted waves, reducing end-to-end reflections,improving and flattening the pass band, suppressing trailing pulses, andother features of improvement to be discussed below.

These and other features of the invention may be particularly understoodwith reference to the drawings in which:

Fig. l is a perspective view of a folded solid delay line constructed inaccordance with the invention;

Fig. 2 is a perspective view of an alternative form of solid delay lineincorporating the invention;

Fig. 3 is a detailed View, partially in section, of the terminal portionof the embodiment of Fig. l;

Fig. 4 is a schematic view of a portion of the folded line embodied inFig. l;

Fig. 5 is a fragmentary view, partially in section, of a portion of lineof the type illustrated Vin Fig. 2, showing a preferred form of surfacetreatment for the suppression of trailing pulses where longitudinalwaves are used and for smoothing the pass band where transverse wavesare used;

Fig. 6 is a fragmentary sectional view of the line of Fig. l, showing apreferred form of surface treatment for improving the transmission oftransverse waves;

Fig. 7 is a fragmentary perspective view of the line of Figs. l and 4,showing another preferred surface treatment for improving thetransmission of transverse waves, and detailsof the construction of thefolded portion of that line;

Fig. 8 is a graph showing the calculated transducer loss in theGO-megacycle range as a function of frequency for lines with and withouta quarter-wave solder bond between the crystal and the solidtransmitting medium and with the crystal operating at the thirdharmonic;

Fig. 9 shows the electrical circuit equivalent of the de lay line of theinvention where the crystal is loaded on one face only;

Fig. l0 is a fragmentary view, partially in section showing details ofthe construction of the end cells and the line of Fig. 2;

Fig. ll is a schematic cross-sectional view, taken as indicated by line11-11 of Fig. l0;

Fig. l2 is a fragmentary sectional view of an alternative end-cellconstruction for the line of Fig. 2; and

Fig. 13 shows the types of waves generated by the incidence of alongitudinal wave on a reflecting surface.

It is to be understood that the embodiments here shown and describedare' illustrative oniy of the invention, and that it is intended thatthe principles may be incorporated in other forms, and that othermaterials may be utilized within the scope of the appended claims.

Proper appreciation of the advantages of the present invention in makingpractical a folded solid line requires consideration of the mechanism otpropagation of ultrasonic waves in a solid rod. Two general types ofpropagation of importance in this connection are possible in a rod whenultrasonic vibrations are impressed on one end thereof: by longitudinalwaves` and by shear, or transverse waves. A solid rod conductsultrasonic vibrations very much as a wave guide transmits microwaves.When longitudinal waves are impressed on one end of the rod, they tendto diverge. When they strike thesurface of the rod, internal reflectionoccurs. ln addition, they generate shear waves, and the result is that aseries of pulses ary rives at the'opposite end of the line. tending tointerfere with clarity of reception. Successive internal reflections Yalong theline produce additional impulses with similar inversions oftype-each incident longitudinal wave gen erating shear waves and in turnbeing reflected, and each incident shear wave generating longitudinalwaves and in turn being reflected.

At the lower frequencies, transverse waves. which have line 29,- whichis mounted therein.

a velocity of propagation about half that of longitudinal waves, appearto possess substantial advantages, as will be discussed in connectionwith the embodiment of Fig. l, which is designed to operate withprincipal vibrations at substantially l megacycles.

At higher frequencies, as for example in an embodiment similar to thatof Fig. 2 operating with the principal vibrations at about 60megacycles, it may be more advantageous to use longitudinal waves, forreasons which will be discussed more fully hereafter.

A third type of propagation, that involving torsional stress, has beenfound useful at frequencies very much lower than those here transmitted.This type is useful within the audible range, for example.

One embodiment of a folded line is seen in Fig. l, in which a mountingbase 1 is shown supporting an input terminal box 2, an output terminalbox 4, and a U-shaped folded solid line 5. Line 5 forms the mechanicalpath l between input and output transverse mode crystal head ssemblies,which are mounted within the terminal boxes, as illustrated in detail inFig. 3.

The solid line 5 is constructed of fused silica, or equivalent materialsuch as the commercially known Corning Glass Works product, Vycor 790.lt is important that the line be free from bubbles and well annealed.Other materials, such as aluminum or magnesium, may be used, but havelimited application, particularly at the higher frequencies. With fusedquartz or glass, the loss is substantially linear with frequency over awide range, whereas with metals it increases rapidly at the higherfrequencies. Lack of grain structure in fused quartz or glass appears toexplain why the loss therein does not similarly increase with frequency.

Line 5 consists of two long parallel legs 6 and 7, joined at one end bya short transverse leg 9. The parallel relation between legs 6 and 7 ismaintained by a transverse brace 10 disposed near the ends 11 and 12opposite the rod portions joined by transverse leg 9. A number ofsupporting blocks 14 are provided at intervals along base 1, and line 5is secured thereto by suitable fittings 15.

The input and output terminal boxes 2 and 4 afford protection andelectrical shielding for the input and output crystal heads 16 and 17,and for the bonds between those heads and the ends 11 and 12 of the linelegs 6 and 7, respectively. The crystals may conveniently be Y-cutquartz, vibrating in the transverse mode. Apertures 18 are formed inboxes 2 and 4 of greater diameter than the ends 11 and 12, so that thelines do not come into contact with the box walls.l Each terminal box,as seen in Fig. 3, is provided with a conventional fitting 19 to which acoaxial cable, not shown, may be connected. Within the terminal boxes,connecting leads 2l) and 21, which may be of the flat ribbon type,extend from the sleeve and center conductor, respectively, of fittings19 to the external connecting electrodes 22 and 24 of the crystal headassemblies 16 and 17. The sleeve leads 2t) are usually grounded. Acommon cover 25 is provided for the terminal boxes 2 and 4.

ln those cases where a shorter delay period is adequate, or wherecompactness is not a factor, the solid line construction may be usedwithout folding, but still using transverse mode vibrations. Such anembodiment is shown in Fig. 2. Mounting feet 26 carry a rigid tube 27,preferably of aluminum or similar light material, to furnish Vthemechanical support for the straight solid delay l A coaxial connector1'9 is secured through each ot' the mounting feet 26. The line 29 iscentered within tube 27 by several guard rings 30, each having threesupporting and adjusting screws 31 spaced equi-angularly about the line.Guard rings 30 alsoY assist in aligning line 29 during assembly. Screws31 are operable from outside the tube 27 and are provided `with lockVnuts v32, as shown in Fig. l0. This straight line construction is moreappropriate for perma- "nently'emplac'e'd structures, since the requiredlength is over eight feet for a line giving a delay of only 672 ,useconds. For shorter delays, portable lines of this design are feasible,using either longitudinal or transverse waves.

The advantages of the folded line in ease of carrying are wellillustrated by a comparison of the somewhat un` wieldly embodiment ofFig. 2 with that shown in Fig. l, which provides the same delay withonly a four-foot over-all length, and is much simpler to supportmechanically than the longer straight line. The same principles may beapplied to the construction of still more compact lines providing thesame delay, by using a plurality of reflecting surfaces and paths. v

An important feature of the present invention which permits folding theline for use with transverse waves as shown in Fig. 1 is in theprovision of 'means for refleeting the transverse waves at the cornersof the bend without distortion or change of wave form. Y Transversewaves propagated in a solid rod are polarized. It has been found that ifthe transmitting and receiving crystals are aligned properly witheachother, and the reilecting surfaces are carefully oriented so thatthey are exactly parallel with the direction of particle motion of thetransverse waves, there will be no loss in the quality of thetransmitted waves because of the reection. In the prior art, severedistortions had been obtained in the output which materially limited theusefulness of the solid line.

In Fig. 4, the folded rods are shown removed from the associatedapparatus which goes to make up the complete folded line. The over-allpath of the ultrasonic waves through the line is indicated by the brokenline 35, with the direction as shown by the arrowheads.

The corner reflectors 36 and 37 are disposed at exactly l5 degrees tothe longitudinal axes of legs 6 and 7, and are ground to secure preciseorientation parallel to the anticipated direction of particle motion.

While this angle has been shown in Figs. l, 4 and 7 as being degrees inorder to produce a simple, symmetrical structure with two parallel legs,reflection of the ultrasonic waves may be accomplished at any otherangie desired, provided the requirement discussed above is met, that is,that the reecting surfaces be oriented properly with relation to thedirection of particle motion.

The correctness of the conclusion expressed above may be veried by amathematical consideration of the total reflection of a transverse wavefrom an air boundary when the direction of particle motion is parallelto the reflecting plane.

The indident wave may be written:

r do e v :L x

w= -sxn o( t-v-l) (l) where u, v and w are the particle displacements inthe x, y and z directions, respectively,

x=distance along x axis, and

vt=velocity of shear wave propagation.

Only one stress, a shearing stress, will exist, and where n=shearmodulus,

traction at the reflecting plane is equal to -xz cos so that theresulting forces at the air-solid boundary are zero,

, indicated in Fig. l2.

as they must be. Hence, proper orientationl of the crystals andreflecting surface will eliminate the objectionable results otherwiseexperienced with folded lines in which transverse vibrations are used.

The next consideration is the smoothing of the pass band of the line,Figs to 7 being illustrative of means for accomplishing this result. InFig. 5, line 6 is shown as having aroughened surface 39. The rougheningis preferably done by chemically etching the rod, but could likewise bedone mechanically. This treatment is likewise effective in the case ofthe 60-megacycle compressional Wave line described hereafter in reducingtrailing pulses. It is believed that the effect in this case is due tothe scattering of the reflected energy to such a degree that no distincttrailing pulses arrive at the receiving end of the line.

In Fig. 6 is illustrated another preferred structure for improving thetransmission characteristics of a line using transverse waves. About thesurface of line 6 is applied a continuous sheet or layer 40 of rubber,gutta percha, acetate cloth, or similar low impedance material.

Fig. 7 shows the application of equivalent material in the form of tape41, which may be more conveniently applied to the folded line. Frictiontape, rubber tape, or the commercially known Scotch tape, apressuresensitive adhesive having a rubber-like base, are all examplesof tape materials which have been used successfully.

By the use of these embodiments the loss characteristics may be smoothedout very substantially over the pass band, so that much more accuratereproduction of the delayed pulses is obtainable, particularly in thecase of relatively long lines. One example, described hereafter,provides a 672 its. delay. Its measured loss variation between 9 and 14megacycles is reduced from :t5 decibels to iLS decibels by wrapping. Itwas found that a l its. pulse could be transmitted with an amplitudedistortion of only 0.1 decibel, and with a reduction in spurious signalsof 40 decibels down from the main pulse at '11.5 megacycles.

In short lines, the surface treatment illustrated in Figs. 6 and 7 maybe effective in suppressing end-to-end reflections. In a particularline, it was found that this surface treatment had no appreciable effecton the transmission of the direct pulse, but first end-to-end reflectionwas reduced by as much as 6 decibels compared to its value with nosurface treatment, and the second end-to-end reflection was reduced byan additional l2 decibels over the useful range.

For both short and long lines, suppression of the endtoend reflectionsor echoes is also materially aided by the use of absorbing end cells, ofwhich preferred forms are shown in Figs. l0 and l2.

In Fig. l0, reflection of waves striking the receiving crystal 34 isreduced by applying to the side of the crystal oppositethat attached tothe line an absorbing end cell 3S, of lead or quivalent material whichhas a high attenuation for the ultrasonic waves. Lead-cadmium andlead-bismuth alloys have also been found very satisfactory.

- The impe-.lance of fused silica is matched by a per cent cadmium, 75per cent lead alloy; and that of 790 Vycor by a 2() per cent cadmium, 80per cent lead alloy, for example.

' Another method of reducing end-to-end reflections is An absorbing cell45 of the same material'as the line 29 is secured to the face ofreceiving crystal 34" opposite thereto. Cell 45 has one beveled face46-dispesed at an angle other than 90 degrees to the ot" reflection atthe crystal face. By this means alone the waves scattered by the beveledface 46 and ultimately impinging on crystal 37 could be reduced inamplitude by only 25 decibels as compared to the waves incident fromline 29. A substantial additional loss was found to be obtainable byadding an absorbing layer 47 to the beveled face 46; if very highattenuation is desired, a second absorbing layer 49 may be secured tothe cell 45 opposite beveled face 46. The material of which layer 47 isformed is lead or one of the alloys of lead and cadmium mentioned above.Other suitable materials may be used. for example, a eutectic of lead,cadmium and tin. With this addition, spurious waves returning to thecrystal 34 could be reduced by more than 40 decibels as compared to thewaves incident from line 29.

In addition to the advantage in reducnig echoes, the use of an end cellot' the same material as the line itself prevents warping and breakingof the crystal due to thermal changes, which is of practical advantagewith the very thin crystals used for high frequencies.

Under conditions where the use of longitudinal waves is desirable, thetreatment illustrated in Fig. 5, in which a roughened surface isproduced on rod 6, as by chemically etching, is found to substantiallyreduce distortion caused by mode conversion at the reflecting boundaryof the transmitting medium. lt is believed that the result is due to thescattering of the reflected portions of the longitudinal wave, so thatlittle energy arrives in phase at the receiving crystal as a perceptiblesignal.

The next feature of importance in improving the transmissioncharacteristics of the line, that of widening and flattening the passband by the use of quarter-wave solder bonds, will be discussed inconnection with the graph shown in Fig. 8, the equivalent circuit ofFig. 9, and a practical embodiment for longitudinal waves similar inexternal appearance to that shown in Fig. 2 for transverse waves.rl'hese figures will be considered as applicable to a line operating atmegacycles, the crystal vibrating at its third harmonic.

ln Fig. 8 are shown curves illustrating the variation with frequency ofcalculated transducer loss in decibels for two lines similar except forthe use in one of a quarter-wave solder bond between line and crystal.Such quarter\vave solder bonds are shown in Fig. 10 at 50 betweencrystal 33 and line 29 and at 5l between line 29 and crystal 34. Theimpedance of the solder is somewhat grcater than that of thetransmitting medium to provide the wide band width shown. It will beobserved that the use of the quarter-wave bond substantially widens andilattens the pass band, or region of relatively low transducer loss.

Particularly at the higher frequencies, as at 60 megacyclesfthe bondbetween the crystal and the fused silic'a rod is of critical importance,and because of the very short wavelengths, cannot be neglected. Itsefect may be seen by a consideration of the equivalent circuit shown inFig. 9 for a crystal loaded on one face only. Approximation by lumpedelements is not possible because'of the large band widths involved.

ln the circuit of Fig. A 9, a one square centimeter crosssectiou isassumed, with c piezoeleetrio constant The equivalent circuit of Fig. 9may be readily solved by conventional mathematical procedures. Thetransducer loss may then be ascertained by comparing the particlevelocity in the transmitting medium obtained with a given inputtermination with that resulting from the use of an ideal transformer tomatch driving generator impedance Z5 to the impedance Zq of thetransmitting medium. The comparison between transducer losses with andwithout the one-quarter wavelength solder bond may then be made as shownin Fig. 9. The use of a one-quarter wavelength thick bond gives asymmetrical band, which widens as the mechanical impedance of thebonding material increases. If the bond is not onequarter wavelengththick, asymmetry results in the band.

While the transducer loss increases as the mechanical impedanceincreases, this may be offset by raising the electrical load impedance,which attcns the band and reduces the over-all loss.

The use of quarter-wave solder bonds to widen the pass band and attenthe response may be applied to liquid as well as solid delay lines.Because of the impedance relations, it has been ditiicult to obtain Wideband widths with liquids other than mercury. With this invention, wideband variable delay lines may be made, wide band liquid cells producedfor studying ultrasonic radiation over a wide frequency range, and avariety of other applications made possible.

One technique of producing the one-quarter wavelength bonds foroperation at 60 megacycles employs a lead-tin-bismuth eutectic solderfor satisfactory' performance. The surfaces to be joined are coated witha layer of silver paste, which may be baked on, burnished,nickel-plated, and tinned. The crystal and rod may then be preheated,the crystal slid onto the rod, and a pressure of about twenty-livepounds per square inch applied. The technique should be modified, ofcourse, to secure the proper thickness required for other frequencies.

The solders used had a somewhat higher characteristic impedance thanthat of the fused silica or other transmitting medium. The best solderwas an eutectic consisting of 32 per cent lead, 15.5 per cent tin, and52.5 per cent bismuth. A second solder consisted of 50 per cent lead, l5per cent tin, and 35 per cent bismuth. Another usable solder consistedof 65 per cent lead, 24 per cent bismuth, 6 per cent tin, and 5 per centcadmium.

Since these were low melting point solders, extremely high stresses werenot set up as the solders cooled, which reduced the danger of cracking.

In summary, the invention as described permits the construction of muchmore compactlines by allowing multiple reflection around corners withoutloss of quality of the transmitted wave; it eliminates end-to-endreflections or echoes which interfere with theprincipal transmittedwaves; it reduces the trailing pulses commonly troublesome withlongitudinal waves, smooths out the pass band characteristic, and makespossible operation over wider band widths for a given transducer loss.

What is claimed is:

1. In a delay line, the combination of an input piezoelectric crystalarranged for vibration in the transverse mode, an output piezoelectriccrystal arranged for vibration in the transverse mode, a solidtransmitting medium connected to said input and output crystals having atransmitting path therethrough defined by a number of reflectingsurfaces formed on said solid transmitting medium and oriented parallelto the direction of particle motion produced in said medium by saidinput crystal.

2. In a delay line, the combination of an input piezoelectric crystalarrangedfor vibration in the transverse mode, an output piezoelectriccrystal arranged for vibration in the transverse mode, a folded solidrod connected to said input and output crystals, and reflecting surfacesformed at thc folded portions of said rod and oriented parallel tothedirection of particle motion produced in said rod by said input crystal.

3. In a delay line, the combination of an input piezoelectric crystalarranged for vibration in the transverse mode, an output piezoelectriccrystal arranged for vibraand 20 per cent cadmium.

tion in the transverse mode; a connection between said input and outputcrystals, comprising a first elongated rod joined at one end thereof tosaid input crystal, a transverse rod disposed normally to said rstelongated rod at the end thereof opposite that joined to said inputcrystal, a second elongated rod disposed parallel to said firstelongated rod and normal to said transverse rod, and reecting surfacesdisposed at the junctures of said transverse rod with said elongatedrods and forming an angle formed of the same material as saidtransmitting medium, l

a beveled surface formed thereon, and an absorptive layer having animpedance substantially the same as that of said transmitting mediumsecured to the free surfaces of said absorbing section including saidbeveled surface.

5. In combination with a delay line having an input piezoelectriccrystal, an output piezoelectric crystal, and a transmitting mediumconnected to said crystals, means for reducing end-to-end reflections,comprising an absorbing section joined to one of said crystals on theside thereof opposite said transmitting medium and being formed ofmaterial having substantially the same imped ance as said transmittingmedium, a beveled surface formed thereon, an absorptive layer having animpedance substantially the same as that of said transmitting mediumsecured to the free surfaces of said absorbing section including saidbeveled surface.

6. An absorbing layer for a fused silica delay line comprising a layerof solder having a high impedance relatively to fused silica, the solderin said layer comprising an eutectic mixture substantially composed of32 per cent lead, 15.5 per cent tin, and 52.5 per cent bismuth.

7. An absorbing section of a delay line lof fused silica or the like,comprising a layer of soider having an impedance substantially the sameas that of fused silica or the like, comprising an alloy substantially75 per cent lead and 25 per cent cadmium.

8. An absorbing section of a delay line of fused silica or the like,comprising a layer of solder having an impedance substantially the sameas that of fused silica or the like, comprising an alloy ofsubstantialty 80 per cent lead References Cited in the tile of thispatent UNITED STATES PATENTS 756,203 Barthel Apr. 5, 1904 1,333,666Luckey Mar. 16, 1920 1,645,098 Friedrich Oct. 11, 1927 1,775,775 NyquistSept. 16, 1930 2,089,492 Lambert Aug. l0. 1937 2,150,530 \Varsirig Mar.14, 1939 2,159,982 Bullock May 30, 1939 2,418,964 Arenberg Apr. 15, 19472,421,026 Haltet al Ma; 27, '1947 2,427,348 Bond Sept.. 16, 19472,430,013 'Hansell Nov. 4, 1947 2,490,452 Mason Dec. 6, 1949. 2,495,740Labin er al Jan. 31,1950 2,503,831 Mason Apr. 11, V1950 2,505,364Mcskimin Apr. 25, 195'0 2,505,515 Arenberg Apr. 25, 1950 2,577,500Arenberg Dec. 4, 1951 2,590,405 Hansell Mar. 25, 1952 2,624,804 ArenbergJan. 6, 1953 2,668,529 Huter Feb. 9, 1954

